Load modulation is an efficiency and linearity enhancement technique which can be subdivided into two major categories: Doherty architectures and dynamic load modulation (DLM). Doherty architectures are based on active load modulation. Dynamic load modulation is based on tuneable components. The Doherty architectures approach suffers from several drawbacks, such as the need for at least two transistor devices with appropriate size ratios, limited RF bandwidth and losses in the combination/impedance inversion circuit. DLM as an alternative technology uses tuneable passive or active components to modulate the amplifier load appropriately.
The efficiency of an amplifier is given by the following equation:
                    η        =                                            i              rms              2                        ⁢                          R              L                                                          i              dc                        ⁢                          V              dc                                                          (        1        )            Where irms is the drain current of the transistor, RL is the real part of the output impedance termination, idc and Vdc are the DC current and voltage supply, respectively. For a given irms and DC supply, RL is chosen such that efficiency is optimised. This leads to suboptimal operation for any other value of irms i.e. under back-off away from peak power. From (1) it follows that by varying Vdc or RL, efficiency can be sustained for any output power level. Varying Vdc leads to dynamic supply modulation (for example envelope tracking or envelope elimination and restoration) while varying RL leads to load modulation (Doherty or DLM). As discussed above, DLM is favourable compared to Doherty architectures.